There are many applications where it is desired to send an audio signal over a relatively long distance. For example, in an auditorium-type audio system, it may be useful to send an audio signal from a source device, such as a microphone, to a far-away output device, such as a speaker, which may be more than 100 feet away from the microphone.
FIG. 1 shows a prior art system 5 that can be used in such an application. As shown, an audio signal is sent from a source device 8 to a transmitter 10. If the amplitude of the audio signal is too low to be suitable for transmission, it can optionally be amplified by an amplifier in the transmitter 10 as shown. Ultimately, the transmitter 10 processes (e.g., filters, amplifies, etc.) the audio signal and drives the audio signal onto a pair of signal wires 14 and 16 that carry the audio signal to a receiver 12. As alluded to above, the signal wires 14 and 16 can be relatively long. Typically, the signal wires 14 and 16 are incorporated into a single cable having a shield (not shown), which shield is typically connected to the chassis ground of the transmitter 10.
Although not shown, the transmitter 10 can comprise many inputs connectable to different sources 8, and can comprise many paired outputs to transmit audio signals to a number of different receivers 12. Moreover, the transmitter 10 may be connected to and controlled by, or incorporated with, a computer 11. Computer 11 allows a user to route the audio signal onto a suitable signal wire pair 14/16 destined for an appropriate receiver 12, and otherwise allows a user to control the processing at the transmitter 10 via software operating on computer 11. Computer 11 may be connected to the receiver 12 as well, or the receiver can be connected to its own control computer, or to no computer at all. Use of no computer at the receiver 12 is common, as appropriate control and processing of the audio signal can occur on the front end at the transmitter 10.
Once received at the receiver 12, the received audio signal can again be processed (again, filtered and amplified), and ultimately output to an output device 18, such as a speaker, where it can be heard. One skilled in the art will understand that the receiver 12 and output device 18 may often be incorporated into a single device or cabinet.
A reality of the use of long signal wires 14 and 16 is increased susceptibility to noise, particularly electromagnetic interference, which may come from many different sources, such as adjacent power lines and other nearby electrical devices. Such noise can cause AC voltage perturbations on one or both of the signal wires 14 and 16 depending on the configuration of the system 5. Noise is obviously not desired because, if not mitigated somehow, it could be received and amplified at the receiver 12, and possibly ultimately heard at the output device 18.
The inventor has seen the need for improved transmitter circuitry that is flexible and configurable to work in different modes depending on the circumstances in which it is used, such as whether the receiver 12 comprises a differential or single-ended device, as will be explained herein. Additionally, the inventor notes that the handling of noise at an improved transmitter should also change depending on these circumstances. An embodiment of such improved transmitter circuitry, and how it operates, is disclosed herein.